Resp:
The combination of 303 and a single 50 cal in each wing of the Mustang MkI was deemed too light for attacking vehicles, etc. When the US Govt submitted/paid for a second order for the British, the RAF requested one change; 20mm cannons, two in each wing (4 cannons). These RAF Mustangs became MkIA. When the US held back the undelivered MkIAs after the attack on Pearl Harbor, these Mustangs became P-51-1 (and -2) which kept the 20mm guns. Almost all of these P-51-1 and -2 were redisginated F-6A, as cameras were fitted for their new role as Photo-Recon aircraft.
Not quite right. The Mark I also had two cheek 50 caliber in addition to the wing armament and was more heavily armed than the A-36, P-51A, P-51B and P-51C. The Mark IA reverted to one of NAA early proposals on the P-509 (the -2) which was forerunner of NA73, which replaced the 4x30 2x 50 wing and 2x 50 cowl guns with 4x20mm Hispano II. That same armament was proposed by NAA for the "Ground Attack Airplane" but AAF changed to 4x50 caliber for the NA-97 A-36. NAA was also requested by RAAF to provide 2x50 cal 2x20mm for the NA-107 which later became NA 110 for Australia but replaced the combined 50 cal/20mm wing battery with six 50's.
When the USAAF placed orders for the A-36A dive bomber, the 4 20mm guns were exchanged for 6 50 cal MG. The two nose guns (50 cal MG) were deleted on the USAAF's first fighter Allison Mustang, which were designated as P-51A. So only wing 50 cal MG, two in each wing, remained. The US govt gave 50 P-51As to the British to replace the MkIAs held back after Pearl Harbor. These 50 were designated Mustang MII, which retained the 4 50 cal MG (no cannons). Sorry. Cannons were not considered better, as they fired much slower than 50 cal MG (from post WWII analysis).
Navalwarrior